Noctiluca
Noctiluca is a genus of dinoflagellates in the family Noctilucaceae. Its only species is Noctiluca scintillans, a marine species that can exist in a green or red form, depending on the pigmentation in its vacuoles. It can be found worldwide, but its geographical distribution varies depending on whether it is green or red. This unicellular microorganism is known for its ability to bioluminesce, giving the water a bright blue glow seen at night. However, blooms of this species can be responsible for environmental hazards, such as toxic red tides. They may also be an indicator of anthropogenic eutrophication.
Etymology
The name Noctiluca scintillans comes from the Latin Noctiluca, meaning "light at night" and scintillans, meaning "shining, throwing out flashes of light".Description
Taxonomy
It was classified with the jellyfish until 1873 when Ernst Haeckel decided to move it to the crystoflagellates with the dinoflagellates. This remained the case until 1920 when Charles Atwood Kofoid finally placed it in the order Noctilucales following certain observations. This classification is still subject to discussion today and the relationship of Noctiluca to the dinoflagellates is not yet clearly demonstrated, as the results of analysis are still too variable to assert a single classification.At present, it is part of the phylum Myzozoa, which are unicellular flagellated organisms. It is then part of the class Dinophyceae, which has two flagella, the order Noctilucales, whose nucleus is not dinokaryonic in the adult, and the family Noctilucaceae, which has a globular shape with a tentacle.
Morphology and anatomy
Noctiluca scintillans is a single-celled spheroid organism, ranging from 400 to 1500 μm in length. It moves with the current and cannot really swim. The fact that it is translucent facilitates the observation. N. scintillans has a long cytoplasmic expansion that hangs at the base of a deep groove, close to which is the nucleus. Another identifying feature are the fine striae that start from the central nucleus and extend towards the periphery of the cell. This species is known by the appearance of blue flashes during night dives. N. scintillans should not be confused with Spatulodinium pseudonoctiluca, which is a similar but smaller species.There are 2 colours of N. scintillans. This depends on the pigment present in the vacuoles. The red form is heterotroph. This form of N. scintillans competes with copepods to feed on phytoplankton. The green form has a photosynthetic symbiont inside called Pedinomonas noctiluca which causes the green color. It is mainly autotroph or even photoautotrophic if this photosynthetic symbiont is abundant in the cells.
Noctiluca scintillans is a species capable of managing its buoyancy by regulating the intracellular ion concentration. To rise, the concentration of potassium will increase and to fall, it will use heavier elements such as calcium or magnesium.
Place in the food chain
N. scintillans has an important place in the pelagic food chain. N. scintillans is preyed upon by many copepods such as Calanus sp., Temora sp. and Acartia sp., chaetognaths and hydromedusae. Because of their excessive proliferation, they attract many predators due to their very dense aggregations and frequent bioluminescence in this phase of their life.The diet varies according to the green and red form. The green form is indeed autotrophic if the symbiont Pedinomonas noctiluca is abundant in its vacuole. Otherwise, it is heterotrophic, like the red form. N. scintillans then feeds on diatom aggregates, as well as copepod eggs, naupilar larvae and fish eggs. Noctiluca scintillans is an interception feeder in which its buoyancy is responsible for its ability to encounter prey and feed successfully. Noctiluca scintillans can remain hungry and live for more than 3 weeks.
N. scintillans can be parasitised by Euduboscquella, an intracellular parasite that infects mainly tintinnids but also dinoflagellates.
Life cycle
Trophonts
Noctiluca scintillans is a heterotrophic dinoflagellate that causes toxic red tides. The life cycle of this species begins as trophonts, which are the non-reproductive adult life stage of many ciliated protozoa. They are eggplant-shaped with a crust consisting of two distinct layers; an outer gelatinous layer and a plasma membrane. Like all eukaryotes, the trophont is composed of a nucleus that lies close to the cytostome surrounded by cytoplasm forming the cytoplasmic center.Gamonts
It is with the gamonts, which is the name of the cells during gametogenesis that cell division occurs. These gamonts are produced by a small fraction of the trophonts that spontaneously initiate gametogenesis. During this transformation, the cell becomes spherical and loses some organelles including the tentacle and the nucleus moves to just below the cell surface.This life cycle continues with two consecutive nuclear divisions to obtain 4 nuclei. This division creates bulges above the cell surface. This is followed by a continuum of synchronous nuclear divisions with each 'progenitor' connected to the others by thin filaments. As gametogenesis progresses, there is a condensation of chromosomes within the different nuclear divisions which darkens the color of the cell. The result is four petal-shaped clusters of progenitors.
Zoospores
The progenitors of the previous stage have transformed into zoospores. At this point they are evenly distributed in one part of the cell. At the same time as the progenitors are maturing, two flagella start to develop and are actively beating. These flagella develop outside the mother cell and the mature gametes are then released into the surrounding environment. When they have all emerged, the mother cell remains ghostly.The two flagella formed are not of the same length and therefore do not have the same function. The longer of the two is used for direction of movement in the sea water, while the shorter one provides more of a swimming force to activate the movement.
Zygote formation
This stage is still highly open to speculation. It seems that Noctiluca scintillans produces isogametes, which are gametes that fuse together to form a zygote. This zygote then has 4 flagella and 2 nuclei. This means that the species is in fact diploid, differentiating it from most dinoflagellates which are haploid.Morphological development from zygote to trophont
At the beginning of trophont formation, the number of flagella decreases and the cells become fusiform. During further development they become rounder, and two distinct flagella are formed, one longer and one shorter, and finally only one is left. After this, the outer layer becomes discernible and the crust is formed. The result is a miniature trophont with a tentacle through which it absorbs food to eat by means of viscous materials to which the algae cling.Thanks to its high specificity, Noctiluca scintillans could increase its biomass up to 100 times in one week.
Environmental influences on sexual reproduction
Recent studies suggest that gametogenesis in Noctiluca scintillans may be related to environmental stress, water quality, temperature, food abundance, environmental changes, and other factors. In most cases, Noctiluca scintillans does not reproduce sexually, and only occurs when the population size reaches a certain concentration, which is greatly related to the food concentration in the environment.When the concentration of food sources in the environment changes dramatically, Noctiluca scintillans will transform from trophonts to gametocyte mother cells, increasing the proportion of gametocyte mother cells in the population. When the food supply in the environment decreases sharply, Noctiluca scintillans may reproduce sexually and produce a large number of gametes as another way of survival after the algal bloom occurs.
Distribution and habitat
Favorable environment
The environment plays an important role in the proliferation of Noctiluca scintillans. The population varies according to sunlight, current, the presence of nutrients, water salinity, temperature and trophic stress. The amount encountered also varies according to the geography and the ocean concerned, although it is present throughout the world.Noctiluca scintillans is found in temperate, subtropical and tropical waters. It is found abundantly close to the coast; it is a neritic species. It is also found abundantly near the mouths of rivers after heavy rainfall. They are mostly found during the warm seasons, although they can be found all year round.
Extreme conditions for the species are 2 to 31 °C and 17 to 45 psu. However, each form has its own preferences and the temperature and salinity ranges are generally more restricted.
The red form is found over a wide temperature range: between 10 and 25 °C and in salty environments. It is very abundant in eutrophic environments where diatoms dominate as this is its favourite food source. The green form is more restricted, with a temperature range of 25-30 °C.
Noctiluca scintillans has consistent spatial and seasonal distribution in the mesoenvironment, with its ability to persist in plankton communities, produce blooms in high concentrations of clematis, accumulate at the surface when the sea is calm, and change its population size according to the water column.
Geographical distribution
Noctiluca scintillans ranges from tropical oceans to northern seas. It is a cosmopolitan species, found in all seas of the world.The green form of N. scintillans is mainly found in the tropical waters of Southeast Asia, the Bay of Bengal, the Arabian Sea, the Gulf of Oman, and the Red Sea. The red form is more widespread, and is found in the seas of Central America, Europe, the Black Sea, East, South and Southeast Asia, and the Tasman Sea. It is also found on the coasts of South America and in the seas of West Africa.
The two forms overlap in the western, eastern and northern Arabian Sea with a seasonal difference in abundance. The green form is found in cold waters, with winter convective mixing, while the red form is found in the warmer summer season.